1 Finnish Meteorological Institute 2 University of Helsinki Helsinki 3 University of Turku

Abstract : The seasonal variation of mono-and sesquiterpene emission rates of Scots pine was measured from April to October in 2004. The emission rates were measured daily in the afternoons with the exception of weekends. Emissions were measured from two branches; one of them was debudded in May branch A, while the other was allowed to grow new needles branch B. The monoterpene emission pattern remained almost constant throughout the measurement period, ?3-carene being the dominant monoterpene 50-70% of the VOC emission. The standard monoterpene emission potential 30°C was highest during early summer in June the average of the two branches 1.35 µg g-1h-1 and lowest during early autumn in September the average of the two branches 0.20 µg g-1h-1. The monoterpene emission potential of branch A remained low also during October, whereas the emission potential of branch B was very high in October. The sesquiterpenes were mainly emitted during mid summer, the dominant sesquiterpene being ?-caryophyllene. Branch A had a higher sesquiterpene emission potential than branch B and the emission maximum occurred concomitant with the high concentration of airborne pathogen spores suggesting a potential defensive role of the sesquiterpene emissions. The sesquiterpene emissions were well correlated with linalool and 1,8-cineol emissions, but not with monoterpenes. Sesquiterpene and 1,8-cineol emissions were equally well described by the temperature dependent and the temperature and light dependent algorithms. This is due to the saturation of the light algorithm as the measurements were always conducted during high light conditions.